1. Field of the Invention
The present invention relates to a main spindle structure of a machine tool, and more specifically, to a tool clamping mechanism of the ball-chuck type with high tool retaining stiffness, and a pull stud used for the tool clamping mechanism.
2. Description of the Prior Art
FIG. 7 is a view showing a main spindle structure that uses a conventional tool clamping mechanism of the ball-chuck type.
A housing 2 is fixed to the distal end of a spindle head 1, and a main spindle 4 is rotatably held by means of a bearing 3 in the housing 2. A draw bar 10 is disposed in a through hole that extends along the axis of the main spindle 4. Balls 7a are embedded in the distal end of the draw bar 10, thus forming a ball chuck portion for holding a pull stud 13 that is attached to a tool shank 5. The pull stud 13 is held by means of the ball chuck portion and pulled into the main spindle 4.
A spring 11 is provided coaxially surrounding the draw bar 10. The draw bar 10 is moved upward as in FIG. 7 by means of the repulsive force of the spring 11, and the pull stud 13 held by means of the ball chuck portion is pulled into the through hole in the main spindle 4.
As a roller 12 advances, the draw bar 10 that is coupled to a guide ring 10b is pushed downward as in FIG. 7. As the draw bar 10 descends, a spread portion of the through hole in the main spindle 4 causes the ball chuck portion of the draw bar 10 to open, thereby allowing the pull stud 13 to be stored. When the pull stud 13 of the tool is inserted into the main spindle 4 through its distal end, the ball chuck portion of the draw bar 10 holds the stud 13.
As the roller 12 retreats, the draw bar 10 is moved upward as in FIG. 7 and pulled in by means of the repulsive force of the spring 11, and the ball chuck portion securely holds the pull stud 13 by means of a contracted portion of the through hole.
When the pull stud 13 is pulled into the through hole of the main spindle 4 by means of the draw bar 10, a taper surface 5a of the tool shank 5 and a taper surface on the inner peripheral surface of the distal end of the spindle 4 are brought intimately into contact with each other. Thereupon, the tool is held integrally with the spindle 4 and clamped.
In general, a disk spring or coil spring is used as the spring 11. The example shown in FIG. 7 is a double coil spring that can generate a clamping force of about 270 kgf. This clamping force is a proper force for the size of the tool shank of the taper-crank type.
The coil spring, compared to the disk spring, is subject to less frictional hysteresis, so that it produces a stable load. Since it smoothly touches the draw bar 10, moreover, the coil spring cannot easily mar the draw bar 10. If it is designed with a stress amplitude not higher than a given value, furthermore, it can repeat the generation of its load substantially permanently, thus enjoying good durability.
As mentioned before, on the other hand, the clamping force is generated by the intimate contact between the taper portions. In order to enhance the tool retaining stiffness, therefore, cuttings or other foreign matter must be kept away from the taper surfaces.
The taper portions can be cleaned by taper cleaning such that foreign matter is discharged to the outside by means of a large quantity of compressed air that is delivered from a center through hole 10c of the draw bar 10 and flows between the taper portions when the taper surface 5a of the tool shank 5 is caused slightly to leave the taper portion of the main spindle 4 by tool changing operation. In attaching a new tool shank to the main spindle 4, their taper portions are subjected to the taper cleaning with compressed air so that they come intimately into contact with each other.
A cutting fluid for cooling and lubricating the cutting edge of a tool can be delivered to the tool through the center through hole 10c of the draw bar 10 and a center through hole in the pull stud 13.
As the productivity of modern machine tools is improved, the tool retaining force is expected to be increased by enhancing the fixation of the tool or tool holder to the spindle shaft, in order to ensure higher cutting performance. To attain this, two-side-restricted tool shanks have started to be generally used (See Japanese Patent Application Laid-Open No. 10-58260).
This two-side-restricted tool shank is designed so that both a taper portion and a cylindrical end face of an arbor are brought intimately into contact with a main spindle. Basically, the tool retaining stiffness of the tool shank of this type can be made higher than that of a tool shank that is designed for intimate contact between taper portions only.
The two-side-restricted tool shank requires cleaning of the contact portions between the respective end faces of the main spindle and the tool shank. If cuttings and the like adhere to the end contact portions, the tool shank is inevitably inclined when it is attached to the main spindle, so that the machining accuracy is lowered. Accordingly, it is necessary to provide means for supplying plenty of compressed air to the end contact portions to blow off the cuttings and other foreign matter, as well as the taper cleaning.
Recently, center-through cooling has been generalized such that cutting oil is supplied from the center of the main spindle to the tool end. In order to feed the cutting oil to the tool end under sufficient pressure according to this method, however, it is necessary to seal the tool shank and the clamping mechanism and provide only the tool end with a fluid channel. This requirement is not compatible with the requirement of the end face cleaning that involves the necessity to deliver air to the taper portions. In general, therefore, an expensive clamping mechanism is needed to reconcile the requirements of the center-through cooling and the end face cleaning.
In order to enhance the tool retaining stiffness for the main spindle, the adhesion between the respective contact surfaces of the main spindle and the tool shank should be enhanced, in ether case of ad hession between only taper portions or case of restriction by two-side ad hesion between a taper portion and end face. To attain this, the tool pulling force must be increased. The tool pulling force of a collet-type clamping mechanism is greater than that of a clamping mechanism of the ball-chuck type.
FIGS. 8A to 8C and FIGS. 9A to 9C show a main spindle structure designed and studied in the process of development of the present invention using the collet-type clamping mechanism. The same reference numerals are used for elements and components which are identical with those in the clamping mechanism of the ball-chuck type shown in FIG. 7.
In this collet-type clamping mechanism, the repulsive force of a spring 11 that acts on a draw bar 10 in a through hole in a main spindle 4 is transmitted to a pull stud 13 after it is amplified by means of a collet 16. FIG. 8A shows a state in which the draw bar 10 is withdrawn in the main spindle. In this state, as shown in FIG. 8C, the collet 16 is closed so that it can clamp the pull stud 13. When the draw bar 10 is pushed downward as in FIG. 8A, resisting the repulsive force of the spring 11, on the other hand, the collet 16 is opened so that it can receive pull stud 13.
A plunger 14, which is incorporated in the draw bar 10, is urged in the upward direction of FIG. 8A by means of a spring 15 in the draw bar 10. When the draw bar 10 is pushed down, as shown in FIG. 9A, the distal end of the collet 16 opens, as shown in FIG. 8B, so that the passage of the pull stud 13 is allowed. When the draw bar 10 is caused to ascend by the repulsive force of the spring 11, as shown in FIG. 9B, on the other hand, the distal end of the collet 16 is closed, as shown in FIG. 8C, and then holds and pulls up the pull stud 13.
In replacing the tool, compressed air is delivered from a through hole 10a in the center of the draw bar 10 and passes through a through hole 14a in the center of the plunger 14 that is urged upward by means of the spring 15. Then, the compressed air is injected into a space in which the collet 16 and the pull stud 13 are to be coupled to each other, and flows between the taper surface 5a of the tool shank 5 and the taper surface of the main spindle, thereby cleaning the taper surfaces. Further, the compressed air passes through a coupling hole 4a in the main spindle, which connects the distal end face of the spindle and the coupling space for the collet 16 and the pull stud 13. Then, the compressed air is jetted out between the respective end faces of the tool shank 5 and the main spindle to be brought intimately into contact with each other, thereby cleaning these end faces.
The repulsive force of the spring 15 is set so that the force of the spring 15 to push up the plunger 14 is greater than the force of the compressed air to push down the plunger 14 during the introduction of the compressed air. Therefore, the plunger 14 is kept upwardly open, as shown in FIG. 9A.
When the clamping is completed with the draw bar 10 raised, as shown in FIG. 9B, the introduction of the compressed air is stopped. When the cutting fluid is introduced through the through hole 10a in the center of the draw bar 10 to carry out machining, its pressure pushes down the plunger 14, resisting the urging force of the spring 15. Thereupon, the plunger 14 is pressed against the distal end of the pull stud 13, so that the through hole 14a in the plunger 14 and the through hole in the pull stud 13 are connected and closed, as shown in FIG. 9C. In consequence, the cutting fluid passes through the through hole 10a in the draw bar 10, through hole 14a in the plunger 14, and the through hole in the pull stud 13, and finally, is jetted out from the distal end of the cutting edge of the tool.
In the main spindle structure of the conventional tool clamping mechanism of the ball-chuck type shown in FIG. 7, the clamping force (pulling force) has the maximum value that can be attained in the actual space even with use of the double coil spring. In order to obtain a greater clamping force, it is necessary to increase the spring coil diameter and enlarge the rear part of the shaft of the main spindle. If a rotating part that has nothing to do with the main spindle stiffness is thickened, the inertia increases, and the acceleration performance of the main spindle worsens. In component machining that entails frequent acceleration and deceleration of the main spindle, therefore, the productivity of the machine may be lowered inevitably. To avoid this, it is necessary to restrict the diameter of the clamping mechanism not larger than the existing value.
With use of the collet-type clamping mechanism with the force amplifying means of FIGS. 8A, 9A, 9B and 9C that is designed to enhance the clamping force (pulling force), it is impossible to restrict the diameter of the main spindle not larger than the existing value, although a designed clamping force is expected to be attained. Supposedly, moreover, the rate of frictional force is so high that the clamping force cannot be stable at the contact portions of the components.
According to the clamping mechanism using a collet chuck, furthermore, the plunger or the like must be provided in the draw bar, as mentioned before, in order to reconcile the center-through cooling and the compressed-air cleaning of the respective contact surfaces of the main spindle and the tool shank. Inevitably, therefore, the resulting structure is complicated and expensive.
The plunger in the central portion of the collet chuck is brought into contact with the distal end of the pull stud under the pressure from the high-pressure cutting fluid. At the start of discharge of the cutting fluid, therefore, the plunger is caused to run against the distal end of the pull stud by a water hammer impact. Thus, there is a little chance for the plunger to fulfill its function steadily for a long period.
It is advisable, moreover, to avoid providing undurable components in those regions in the main spindle which are not accessible to maintenance operation.
For these reasons, the collet-type clamping mechanism is not a favorable mechanism for reconciling the center-through cooling and the compressed-air cleaning of the respective contact surfaces of the main spindle and the tool shank.